1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) used as a display means for computers, audio and video apparatuses, and the like.
2. Description of the Related Art
In recent years, the application of liquid crystal panels utilizing the electrooptic properties of a liquid crystal material to office automation apparatuses and the like has progressed vigorously by achieving a large screen size and a large capacity. At present, liquid crystal display panels generally available in the market employ a twisted nematic (TN) type operation mode, where the orientation directions of liquid crystal molecules in a liquid crystal layer are twisted by 90.degree. between two glass substrates. In such a TN type operation mode, a light transmission intensity varies depending on the viewing direction when a voltage is applied, causing asymmetry of the viewing angle characteristics. The asymmetry of the viewing angle characteristics raises a problem especially in the case of gray-scale display. That is, an extreme reduction in contrast ratio occurs or the contrast ratio is inverted (e.g., positive to negative) depending on the viewing direction, resulting in reducing the display quality.
In order to overcome the above problem, in recent years, attempts for increasing the viewing angle of TN-type liquid crystal display panels have been intensively performed. For example, a technique disclosed in SID 93 Digest, p. 622 (1993) attempts to increase the angle of visibility in the following manner: Alignment films for controlling the orientation of liquid crystal molecules are not rubbed to allow the liquid crystal molecules to randomly orient, thereby forming multiple domains having different orientation directions. This technique is called "random orientation TN". According to this technique, a liquid crystal material is sealed in a space between a pair of substrates, to the inner surfaces of which polyimide alignment films are applied without rubbing, at a temperature equal to or more than a nematic-isotropic phase transition temperature. The liquid crystal material is then cooled, to allow liquid crystal molecules to randomly orient. In this way, multiple domains (microdomains), among which the orientation directions of the liquid crystal molecules are different, are formed, so as to increase the viewing angle.
Hereinbelow, the display method of a liquid crystal display panel having randomly oriented liquid crystal molecules without rubbing will be described with reference to FIG. 10.
FIG. 10 is a schematic view illustrating the orientation state of non-rubbed randomly oriented liquid crystal molecules of a liquid crystal display panel observed when no electric field is applied. This liquid crystal display panel includes an upper glass substrate 201 and a lower glass substrate 202 which have non-rubbed polyimide alignment films (not shown) facing each other with a predetermined space therebetween. A chiral nematic liquid crystal material 207, which has a spontaneous twisting angle of about 90.degree. between the glass substrates 201 and 202 and a positive dielectric anisotropy (P type), is sealed in the space between these substrates at a temperature equal to or more than a nematic-isotropic phase transfer temperature, and then cooled to room temperature to form a liquid crystal layer 220. In this way, liquid crystal molecules 217 and 227 located at the interfaces with the glass substrates 201 and 202, respectively, are oriented completely randomly with an equivalent probability among respective liquid crystal domains (microdomains) 210 at the interfaces with the glass substrates 201 and 202. In each of the liquid crystal domains 210, however, the liquid crystal molecules 217 located at the interface with the upper glass substrate 201 and the liquid crystal molecules 227 located at the interface with the lower glass substrate 202 are oriented in the states twisted by 90.degree. from each other.
In such a liquid crystal display panel, a liquid crystal molecule 237 located in a mid-plane defined by a plane assumed to be positioned in the center of the liquid crystal layer 220 interposed between the substrates 201 and 202 in the thickness direction thereof in parallel with the substrates, is oriented substantially horizontally with respect to the substrates when no voltage is applied. When a voltage is applied, the liquid crystal molecule 237 is gradually tilted so that a dielectric free energy is reduced, i.e., so that the major axis of the liquid crystal molecule 237 is aligned with the direction of the electric field (generally, the direction vertical to the surface of the substrate) if the liquid crystal material has a positive dielectric anisotropy. As the applied voltage is increased, the liquid crystal molecule 237 gradually rises in the vertical direction. In the case where the liquid crystal material has a twisting angle of 90.degree., since the liquid crystal molecule 237 in the mid-plane is located in the middle between the upper and lower glass substrates 201 and 202, the twisting angle of the liquid crystal molecule 237 in the mid-plane is just a half of the total twisting angle, i.e., 45.degree. which is a half of the twisting angle formed by the liquid crystal molecules 207 located at the interface with the upper glass substrate 201 and the liquid crystal molecules 227 located at the interface with the lower glass substrate 202.
The tilt direction of the liquid crystal molecule 237 in the mid-plane when a voltage is applied determines the direction of the viewing angle characteristic. Thus, although one liquid crystal domain 210 has a certain viewing angle characteristic, since a sufficiently large number of liquid crystal domains 210 having random orientation directions of liquid crystal molecules exist in one pixel region, the viewing angle characteristics of the liquid crystal domains are macroscopically averaged. As a result, the light transmittance in various viewing directions become substantially symmetric, causing a reduction of the viewing angle dependence of display quality, including a reduction or inversion of a contrast ratio.
A technique for further increasing the contrast by applying the random orientation method described above to a vertical orientation mode is disclosed in Japanese Laid-Open Publication No. 7-84260. In this technique, a liquid crystal material having a negative dielectric anisotropy (N type) is used for a liquid crystal layer, and vertical alignment films are formed on two substrates, so as to realize a normally-black mode where liquid crystal molecules in the liquid crystal layer are oriented substantially in parallel with the substrate surfaces when a voltage is applied. FIG. 11 is a perspective view illustrating the microscopic orientation state of liquid crystal molecules in one pixel realized by this technique.
Referring to FIG. 11, the reference numeral 253 denotes an orientation direction (director) of a liquid crystal molecule. A plurality of microdomains 254, 255, 256, and 257 having different directors at the substrate surfaces and thus at the mid-plane in one pixel region are formed. As used herein, the expression that "the orientation directions of liquid crystal molecules are different among microdomains or liquid crystal domains" means that the average orientation directions of a plurality of liquid crystal molecules in respective liquid crystal domains, i.e., the average azimuthal directions of the major axes of liquid crystal molecules in respective liquid crystal domains are different among the liquid crystal domains in the mid-plane which is parallel to the surfaces of the substrates sandwiching a liquid crystal layer. A disclination line 258 is observed at a boundary of adjacent microdomains since the adjacent microdomains have different orientation directions.
A technique for further improving the contrast and the display quality obtained in the random orientation method described above is disclosed in Japanese Laid-Open Publication No. 9-73084. FIG. 12A is a sectional view of a liquid crystal display device employing this technique, and FIG. 12B is a partially enlarged perspective view of the liquid crystal panel, schematically illustrating the orientation state of liquid crystal molecules in liquid crystal domains (microdomains).
The liquid crystal display device shown in FIG. 12A includes an upper glass substrate 101 and a lower glass substrate 102 in parallel with each other interposing a predetermined gap therebetween. Each of the glass substrates 101 and 102 includes a transparent electrode 103 on one surface and a polarizing plate 106 on the opposite surface. The transparent electrodes 103 are located on the inner surfaces of the glass substrates of the opposing glass substrates, while the polarizing plates 106 are located on the outer surfaces of the glass substrates. The two polarizing plates 106 are disposed on the outer surfaces of the glass substrates 101 and 102 of the panel so that the polarization axes thereof cross each other at about 90.degree..
Microscopic unit liquid crystal cells (i.e., liquid crystal domains) 110 are formed between the glass substrates 101 and 102. The liquid crystal domains 110 are enclosed by polymer walls 108 standing substantially vertical to the substrates, to be divided from one another. A nematic liquid crystal material 107 having a positive dielectric anisotropy (hereinbelow, occasionally simply called a liquid crystal material) is sealed in the respective liquid crystal domains 110. The reference numerals 104 and 105 denote a seal member and a spacer bead, respectively.
Referring to FIG. 12B, a liquid crystal domain 110a includes liquid crystal molecules having orientation directions (directors) 117, 127, and 137. In this example shown in FIGS. 12A and 12B, the liquid crystal molecules have a 90.degree. twisted orientation along a direction normal to the substrates. Within one liquid crystal domain 110a, the orientation directions 117, 127, and 137 of the liquid crystal molecules are substantially uniform in the corresponding planes parallel to the substrates (e.g., the mid-plane 111). However, they are random among different liquid crystal domains (e.g., liquid crystal domains 110a, 110b, and 110c). Accordingly, by providing a sufficiently large number of liquid crystal domains 110 in one pixel region, the orientation directions of liquid crystal molecules are macroscopically averaged through the entire liquid crystal panel. This makes the transmittance intensities in various observation directions substantially symmetric, and thus improves the viewing angle dependency.
A plasma addressed liquid crystal display device (PALC) has been developed as a large-size liquid crystal display device exceeding a 20-inch type, in place of a TFT-LCD, for use in a future wall type television set and the like. Techniques for realizing the PALC are disclosed in Japanese Laid-Open Publication No. 1-217396 and No. 4-285931. As a technique for increasing the viewing angle of a TN type liquid crystal display panel, Japanese Laid-Open Publication No. 7-120728 discloses an axially symmetric aligned microcell mode (ASM mode) where liquid crystal molecules are oriented axial symmetrically in each pixel region. As another technique for increasing the angle of visibility of a liquid crystal cell, Japanese Laid-Open Publication No. 9-197384 discloses a technique where the above-mentioned ASM mode is applied to the plasma addressed liquid crystal display device described above.
FIG. 13A is a sectional view of a typical plasma addressed liquid crystal display device 300 operating in the ASM mode, and FIG. 13B is a plan view illustrating one pixel region of the plasma addressed liquid crystal display device.
Referring to FIG. 13A, the plasma addressed liquid crystal display device 300 has a flat panel structure including a display cell 301 and a plasma cell 302. The display cell 301 displays images by modulating incident light to output light according to pixel signals. The plasma cell 302 scans (addresses) the display cell 301. The display cell 301 and the plasma cell 302 share an intermediate sheet 303.
The plasma cell 302 includes stripe-shaped discharge channels 305 arranged in a row direction for sequentially discharging plasma to scan the display cell 301 in a line-sequential manner. The discharge channels 305 include partitions 307 for defining respective spaces of the discharge channels 305, anode electrodes A located at the bottoms of the partitions 307, and cathode electrodes K located in the middle between the adjacent anode electrodes A. The anode electrodes A and the cathode electrodes K relate to opposite electrical polarity from each other and space apart from each other. The anode and cathode electrodes, made of a material which does not transmit light, define physical apertures therebetween, to allow light incident on the liquid crystal display device to pass through only these physical apertures.
The display cell 301 includes stripe-shaped signal electrodes 310 arranged in a column direction so as to cross the row direction in which the discharge channels 305 are lined. Pixel regions are formed at the spatially crossing region of the discharge channels 305 and the signal electrodes 310. The signal electrodes 310 apply image signals to a display medium layer 309 in synchronization with the line-sequential scanning, to modulate the incident light for each pixel. The display cell 301 further includes section walls 317 formed in a lattice shape. The section walls 317 serve to regulate the orientation of liquid crystal molecules in liquid crystal regions defined by the section wall 317 so as to be axial symmetric.
The plasma cell 302 includes a glass substrate 304 and is bonded to the back surface of the intermediate sheet 303, while the display cell 301 includes a glass substrate 308 and is bonded to the top surface of the intermediate sheet 303. For example, a liquid crystal material is enclosed in a space between the glass substrate 308 and the intermediate sheet 303 to form the display medium layer 309. A color filter 313 is formed on the inner surface of the glass substrate 308.
Referring to FIG. 13B, the display medium layer 309 is divided into liquid crystal regions 315 which are surrounded by the section walls 317 formed in a lattice shape. The respective liquid crystal regions 315 are defined by the section walls 317 with respect to the positions and sizes thereof. Liquid crystal molecules in the respective liquid crystal regions 315 are controlled to be oriented axial symmetrically by the alignment force of the surfaces of the section walls 317. In FIG. 13B, the cathode electrode K is shown to be disposed in the middle of the discharge channel 305 with the anode electrodes A disposed on the sides thereof. Two physical apertures spaced apart from each other are therefore formed in one pixel region 311. The portion where each of the physical apertures of the plasma cell 302 and each of the liquid crystal regions 315 overlap each other defines an aperture which contributes to display. In FIG. 13B, two liquid crystal regions 315 are formed in one pixel region 311 so as to correspond to the two physical apertures.
The above-describe prior art techniques have respective problems as follows.
In the technique disclosed in SID 93 Digest shown in FIG. 10, liquid crystal molecules in the liquid crystal layer are oriented substantially in parallel to the substrate surfaces when a voltage is not applied. When a voltage equal to or more than a threshold value is applied, the display device operates in a normally-white mode where the liquid crystal molecules are oriented in a direction substantially vertical to the substrate surfaces. In this state where a voltage equal to or more than a threshold value is applied, a disclination line is generated at the boundary of liquid crystal domains having different initial orientation states. Therefore, according to this technique, although the angle of visibility increases, the disclination line is displayed as a bright line when an image is observed in a tilt direction, generating light leakage and thus failing to obtain a sufficiently high contrast. Moreover, a high driving voltage is required due to the disclination line.
In the technique disclosed in Japanese Laid-Open Publication No. 7-84260 shown in FIG. 11, the sizes of the microdomains 254, 255, 256, and 257 are not restricted. If the number of microdomains in one pixel region is not sufficiently large, it may not be possible to secure a sufficiently large number of microdomains where the orientation directions of liquid crystal molecules are different among the microdomains so as to provide the state in which the liquid crystal molecules are randomly oriented through the entire pixel region. If the orientation directions of liquid crystal molecules do not obtain random orientation through the entire pixel region, the viewing angle dependency remains. Moreover, if the size of the microdomains is not sufficiently small, the difference in the transmittance intensity among the microdomains is observed as an uneven display, markedly reducing the display quality.
In the technique disclosed in Japanese Laid-Open Publication No. 9-73084 shown in FIGS. 12A and 12B, the polymer walls substantially vertical to the substrate surfaces are formed around the respective microdomains to control the size of the microdomains. A number of polymer walls are formed in each pixel region. If such polymer walls are light-blocking walls which do not transmit light, they exist as regions which cannot contribute to display during a white display, reducing the aperture ratio of the display device. If the polymer walls are transparent, bright spots are generated due to light leakage through the polymer walls during a black display. Moreover, since this technique employs the normally-white (NW) mode, the problem described in relation with the technique disclosed in SID 93 Digest arises. That is, when a voltage equal to or more than a threshold value is applied, a disclination line is generated at the boundary of liquid crystal domains having different initial orientation states. Therefore, although the angle of visibility increases, the disclination line is observed as a bright line in a NW mode when an image is observed in a tilt direction, generating light leakage and thus failing to obtain a sufficiently high contrast ratio. Moreover, a high driving voltage is required due to the disclination line.
In the plasma addressed liquid crystal display device disclosed in Japanese Laid-Open Publication No. 9-197384 shown in FIGS. 13A and 13B, the portion where each physical aperture formed between the anode electrode A and the cathode electrode K of the plasma cell and each liquid crystal region of the display cell overlap each other defines an aperture which contributes to display. In such a plasma addressed liquid crystal display device, the plasma cell and the display cell are fabricated separately and then bonded together to complete the device. In order to secure an aperture having a designed area, therefore, the plasma cell and the display cell must be precisely aligned with each other when they are bonded. However, since the cathode electrodes K, the anode electrodes A, and the partitions 307 are formed by a printing process, the patterning precision is as low as about .+-.10 .mu.m. Accordingly, in order to secure the designed area of the aperture, a large alignment margin is required. This increases the area of regions which cannot contribute to display and reduces the aperture ratio of the display device. Moreover, since precise alignment is required, the production yield reduces and thus the production cost increases.